1. Field of the Invention
This invention relates, in general, to apparatus for precisely mixing liquids in high pressure systems and, more particularly, relates to solvent gradient generating apparatus in liquid chromatography systems.
Liquid chromatography pertains to a particular variety of equipment and techniques for analyzing the components of an unknown sample of liquid material, qualitatively and/or quantitatively. According to one liquid chromatographic system, a column cell is provided having a stationary liquid contained therein such that an immiscible mobile liquid percolates or flows therethrough. Such a liquid/liquid chromatography system may be characterized as a partition or solution system. The sample is retained by partitioning between the mobile liquid and the stationary liquid.
In another liquid chromatographic system, the column contains a solid instead of a stationary liquid. Such a system may be characterized as an absorption chromatography system. The mobile liquid is propelled through the solid absorbents, and variations in properties of the substance emerging from the column are detected and plotted.
In chromatography systems, mobile liquid flow rates and volumes must be precisely controlled if accurate analysis is to result. Constant flow rate systems are utilized for minimizing adverse affects of changes in uncontrolled variables on the constancy of the volumetric flow rate. Two primary classes of pumps are utilized in a constant flow rate system. A displacement type pump expels fluid contained within a limited reservoir by a piston under pressure. A displacement type pump advantageously provides nonpulsating flow, but provides only a limited amount of liquid due to its finite capacity. A reciprocating type pump, however, provides an unlimited reservoir and therefore an unlimited amount of liquid; but its flow is undesirably pulsating. Pulse damping apparatus must accompany reciprocating pump systems because detectors are usually flow rate sensitive. Detector noise levels are often determined by short term pulsations in flow from these reciprocating pumps. However, pulsating pumps are less expensive than displacement type pumps. It is therefore economically desirable to utilize the pulsating type pumps in combination with pulse damping apparatus.
Several pumping schemes have been proposed whereby solutions are mixed in precise amounts with respect to time. One proposed gradient generator utilizes a pair of high pressure displacement pumps which dispels fluid into a common conductor, such as a tee or mixing chamber, at controlled rates. Such a system however requires two high pressure pumps and is therefore relatively expensive.
Another pumping system heretofore proposed utilizes a first high pressure pump propelling its solvent into a second high pressure pump where the solvent is stirred with the contents of the second pump. The second pump acts as a variable volume mixing chamber to provide the desired mixture. As with the proposed system described above, a pair of expensive high pressure pumps is required.
Another proposed pumping system utilizes a low pressure metering pump dispelling its liquid into a calibrated volume of solvent of different composition. The mixture of solvent initially in the calibrated volume with liquid pumped in by the low pressure metering pump forms the feed solvent for the high pressure pump. The high pressure pump must have a small hold-up volume. Low hold-up volume, high pressure pumps are of the reciprocating type. An undesirably pulsating flow to the column is provided, unless pulse damping apparatus is supplied. Accordingly not only are two pumps required, but damping apparatus is also required.
Still another contemporary pumping arrangement in chromatography systems use a single high pressure pump in combination with a storage coil, a mixing tee, and a pair of solenoid valves coupled to the mixing tee. Each solenoid valve is alternately pulsed into the open position, and the relative time for each of the valves to open determines the mixture of the final solution delivered into the mixing tee. However, such pulse solenoid valves typically exhibit a relatively short life cycle. Furthermore, certain liquid combinations pose an unacceptable mixing problem at the mixing tee when passing through the valves. Still further, because such a system mixes the two components in the storage coil, part of the liquid solvent at the interface between the two components must be discarded.
It is therefore a general object of the present invention to provide an improved low-cost, high-pressure and constant volume flow rate solvent gradient generator in a chromatography system.